For a long time, wireless communication systems have been facing a conflict between the limited spectrum resources and the continuously and quickly increasing of the number of users. Although system capacity has been increased to a certain extent by technologies such as frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA), these technologies are far from the demand of ever-increasing quantity of wireless traffic. Therefore, people begins to utilize the spatial domain characteristic of a data transmission channel, e.g. diversity, sectorization, and switching multi-beam and adaptive antenna array, etc., to increase the capacity of a receiving system. By using these methods, communication quality of a wireless communication system have been improved to some extent, and the capacity have been increased.
In the diversity technology, signals received by different antennas with a space larger than 10 carrier-wavelength are not correlated. The signals received by respective antennas are combined by using a maximum ratio to improve system performance such as multipath anti-fading.
In the sectorization technology, a cell is divided into 3, 6, 9, or 12 sectors, each sector being configured with an antenna and a predetermined spectrum range. The interference of the communication channels can be decreased to a certain extent by sectorization, thereby increasing communication quality of the system.
The switching multi-beam technology is to form fixed beams in a cell in different directions, wherein signal quality of an expected signal in each beam is detected by a base station, and the best beam is selected to be received. One of the main reasons for calling it switching multi-beam is that during a procedure that the system selects a beam, there are controlled switches on the channels between each beam and the respective channel receiver, that is, a “switch matrix”. After a certain beam is selected, a switch between the beam and the corresponding channel receiver is closed, while the switches of the other channels are open.
The signals received by respective antennas are weighted and combined adaptively by an adaptive antenna array based on maximum signal to noise ratio criterion, maximum likelihood criterion, and minimum mean square error criterion, etc. The interference and noise signals are suppressed effectively, thereby increasing the entire performance of the wireless system.
Because the diversity method requires a large distance between the antennas (normally, larger than 10 wavelengths), the more the antennas, the more the spaces are occupied. However, spaces used practically by a base station are limited. In addition, although the diversity method for combining by using the maximum signal to noise ratio has the effect of multipath anti-fading, it cannot suppress signal interference effectively.
The common sectorization methods have used 3 sectors or 6 sectors. The reason for not using more sectors is that the more the division of the sectors, the less the spectrum resources can be used by each sector, and signal relay efficiency will be decreased. Furthermore, the more the division of the sectors, the more the beams overlap between different sectors, interference between the channels will increase, and system performance will decrease.
It can be considered to some extent that switching multi-beam is a type of sectorization method, whereby the division of sectors is formed dynamically by a combination of different beams. Because “the best” beam is set for receiving signals, switching multi-beam differs from the sectorization method. The more the beams of the switching multi-beam overlap, the more gain loss at the boundary between the beams will be decreased. The field of a beam of a current switching multi-beam system is cohered and overlapped by a directional antenna or by using a radio frequency phase shift network (for example, Butler matrix) to form a plurality of narrow beams directing to different directions in the space for covering a cell. In theory, the narrower the beams are, the better the performance of spatial domain filtering a multi-beam antenna switched by beams will have, and the stronger the ability for suppressing the signal interference. However, because an aperture of a directional antenna is limited, and phase shift accuracy of a radio frequency phase shift network is limited, the width of a beam is limited, and the overlapping the beams is limited. As a result, the ability for improving communication capacity of the existing switching multi-beam systems is limited. Furthermore, since a switching matrix of the existing switching multi-beam systems is implemented by radio frequency switching devices, it makes the cost of system hardware increase. When beams are selected by switching beams, it is implemented normally based on power magnitude of an expected signal in a beam. When there is strong interference, and the time for evaluating expected power of a user is rather short, it will sometimes cause malfunction in selecting the beams.
An adaptive antenna array employs an adaptive algorithm based on different standards to obtain an array-weighting factor. Although optimum system performance can be achieved in some extent, a large amount of calculation will be required for the adaptive algorithm with excellent performance. The requirement for digital signal processing devices is rather high, and a number of algorithms cannot be implemented by using high speed processing chips that are currently used.
Based on the above reasons, and by incorporating the technology of switching multi-beam and adaptive antenna array, one of the objectives of the invention is to provide a digital baseband spatial domain matched filtering method for an array receiver in a radio communication system. The method allows simple and small amount of calculation. Therefore, the cost for implementing the hardware is low, while the system performance is better.